The invention relates to an automatic focussing apparatus utilizing the principle of a range finder and which can find applications in a variety of cameras.
The present applicant has previously proposed a variety of automatic focussing apparatus which operates on the principle of a range finder. Specifically, reflective light rays from an object being photographed, which is made incident through a pair of independent windows, are led to a pair of light receiving elements through reflections by a stationary and a movable reflecting mirror. The coincidence of the amount of reflective light is detected and used as a reference for driving a focussing lens. One such apparatus is disclosed in U.S. Patent Application Ser. No. 856,038 filed Nov. 30, 1977 and assigned to the same common assignee as the present invention.
Since the apparatus operates to compare the respective amount of reflective light which are admitted through the pair of independent windows, the determination of the coincidence between both the incident inputs requires the presence of a difference therebetween of a given magnitude below which the detection is impossible. As a consequence, a contrast, of an object being photographed, which is above a preselected threshold is required for the effective operation of such apparatus.
More specifically, when a cine camera incorporating the automatic focussing apparatus disclosed in the above-mentioned patent application is used to take pictures of two men standing in front of a white wall by a panning process, the automatic focussing operation is achieved if either man appears within the field of sight of the camera or the apparatus. However, when only the white wall appears within the sight in the course of moving from one man to the other during the panning process, the focussing operation is disabled because of the loss of contrast in the object being photographed. Consequently, the focussing lens may be moved in an erroneous manner to result in an out-of-focus condition when either man appears again within the field of sight later, thus requiring a repeated focussing operation.